Copending application Ser. No. 07/503,297, now U.S. Pat. No. 5,110,089 issued May 5, 1992, shows an extendible jack stand including a first or bottom frame, second and third elongated upper frames that are normally arranged in telescoping relationship, and releasable locking means that is cam-actuated for automatically locking or unlocking the third frame relative to the second frame. The releasable locking means is quiescent while the third frame is being raised or extended, but when the third frame reaches its limit of extension is cam-actuated to automatically lock the third frame to the second. During retraction, the second frame remains locked to the third frame until the second frame has been fully retracted into the first frame.
As shown in the copending application, it is desirable to maintain the relative rotational alignment of the frames so that none will rotate about its longitudinal axis relative to another, and also to positively lock the adjacent frames at their limits of longitudinal movement relative to each other so that they cannot be inadvertently pulled apart.
Experience has shown that in certain situations the locking action both for rotational alignment and for the limits of extension needs to be very positive; for example, where a misalignment between the associated power unit and the jack stand has occurred, the power unit might overpower the jack stand and pull it apart, unless it is very positively locked.
Thus it is important to achieve optimum operation of the alignment means for preventing relative rotation of the frames; of the locks for limiting the longitudinal extension positions of the frames; and of the releasable locking means that automatically locks or unlocks the third frame relative to the second frame.